This will pop in-game once you get ten different medals. (Yes, make sure they're different.) You can get these in custom matches, like Slayer on a small map.
I played a 4-player game and got medals like First Kill, Grenade Stick, Running Riot, Rampage, Melee, Assassin (I think), multi-kills, etc. There are obviously a lot more than you can get. Just make sure you have enough time/kills so you don't end it early. (I played CTF with a score limit of 1, then got the cheeves and finished the game for the win.